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Bushfire Behaviour

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Published: Sunday, 28 December 2014 11:22

Written by Lance Hartley

(Courtesy of CSIRO Bushfire Behaviour and Management Section)

Heading Fire

A heading fire is one where the flames are blown towards unburnt fuel. The fuel bed is ignited at the top and burns progressively down into the lower layers. A heading fire, particularly under extreme conditions, can be quite inefficient in its combustion, resulting in thick black smoke and partially burnt fuel. Large envelopes of burning gas can be quite often seen as flashes of flame (secondary combustion) well above the average flame height.

Flanking Fire - The edge of a flanking fire is generally aligned parallel to the direction of the wind. Flames more or less lean along the flank. Due to the ever changing nature of the wind, slight changes in wind direction means that the flank will become, by turns, a heading fire and a backing fire in response to the changes in wind direction. Therefore a flank may exhibit the high flames and black smoke of a heading fire one moment, and the low flames and little smoke of a backing fire the next.

Backing Fire - A backing fire is one which moves into the wind. The flames lean over already burnt ground and ignite the fuel at the bottom of the fuel bed. The rate of spread of a backing fire is quite slow and independent of the wind speed. Combustion is often very efficient and complete, resulting in less smoke than a heading fire and, in some fuel types, a fine white ash residue. At different times in the development of a fire, heading fires, backing fires and flanking fires may occur at any location around the fire perimeter, depending on the fluctuations in the direction of the local wind at the fire edge.

The local wind results from the interaction between the prevailing wind and the convective updraft of the whole fire. During a lull in the prevailing wind, the convection from the fire may draw wind towards the fire centre so that a backing fire may occur around the entire perimeter.

Fire Types

Ground fires: burn subsurface organic fuels by smouldering combustion. They can be ignited by surface fires. Organic soils such as peat found in artic tundra, bogs and swamps are prone to these types of fires.

Crown fires: burn through tree crowns and are invariably ignited by surface fires. These can move very fast. Spotting is important here. Some of the major forest fires in southern and eastern Australia have been crown fires. Boreal forests are one of the habitats most prone to crown fires. In Australia some of the most severe fires in southern eucalypt forests can become crown fires.